This invention relates generally to security passageways, and particularly to security doors, and the sensing of unauthorized passage of objects, as well as people through the doors, especially "piggybacking."
Security doors are used in airports, banks, commercial buildings, military installations, and other locations where restricted access is desirable. One type of security door is a revolving door such as disclosed in U.S. Pat. No. 4,627,193. Normally, in this type of door, a person inserts a pass card, then enters a compartment on one side of the door. If the card is authorized, the door will turn until the entered compartment moves from the entrance to the exit. As the entered compartment revolves, all compartments move by a corresponding amount. Therefore, it is possible for an unauthorized person to "tailgate", i.e., to either enter the compartment immediately following the one containing the authorized person, or enter a compartment located at the exit. It is also possible for two people to attempt to pass in the same compartment ("piggybacking").
One way that tailgating and piggybacking have been detected is by the use of floor mats in the security door to detect when a compartment has been entered. However, such mats have several drawbacks.
It has been proposed to use ultrasonic sensors, instead of mats, but use of such sensors in a revolving door presents difficult problems. First, the sensors must distinguish between door panels and people or objects. If the sensors are merely turned off when the door panel passes by, objects attached to, or close to, the door panel can get through the doorway undetected. Second, to detect small objects, such as pass cards or firearms, the sensors must have a high gain. Such a high gain increases the likelihood that reverberations or echoes will cause false readings. This is especially true in a security revolving door which has a substantially closed housing. Similarly, the greater the range (portion of the floor to ceiling distance) covered by the sensor, the greater the likelihood of false readings due primarily to echoes from the floor. Moreover, piggybacking has been undetectable with ultrasonic sensors. Accordingly, in spite of their drawbacks, floor mats have been popular in security door devices.